1. Field of the Invention
The present invention relates to an objective lens driver for use in an optical disc drive for reading and/or writing information optically from/onto a disk storage medium by focusing a light beam thereon.
2. Description of the Related Art
An optical disc drive for reading and/or writing information optically from/onto a disk storage medium such as a compact disc (which will be simply referred to herein as an xe2x80x9coptical discxe2x80x9d) includes an optical head. The optical head moves in the radial direction of the disc, thereby focusing a light beam onto a predetermined track to read or write information therefrom/thereon. Also, the optical head detects the light beam that has been reflected from the disc and converts the light detected into an electric signal.
The optical head includes a light source and an objective lens for use to focus the light beam that has been emitted from the light source. The optical head drives the objective lens such that a beam spot, formed on the disc by the light beam focused thereon, follows predetermined tracks on the disc while maintaining a constant focusing state. More specifically, the objective lens is driven both perpendicularly and parallelly to the information recording side of the disc in such a manner as to correct a focus error and a tracking error, which may be caused by the flutter and eccentricity of the disc, respectively. The former direction that is perpendicular to the information recording side of the disc is parallel to the optical axis of the objective lens, and will be referred to herein as a xe2x80x9cfocusing directionxe2x80x9d. On the other hand, the latter direction that is parallel to the information recording side of the disc is the disc radial direction, and will be referred to herein as a xe2x80x9ctracking directionxe2x80x9d. Such a mechanism which is specially designed to drive the objective lens will be referred to herein as an xe2x80x9cobjective lens driverxe2x80x9d. It should be noted that the objective lens herein forms an integral part of the objective lens driver.
Recently, the optical discs should have even higher storage capacities and the optical disc drives should achieve even higher transfer rates year after year. To meet these demands, it has become increasingly necessary to perform even more precise positioning control on the objective lens and transfer information, which has been read out from, or is going to be written on, an optical disc rotating at a high speed, from/to the optical disc at an even higher rate.
When the optical disc is rotated at a high speed, the acceleration of the flutter and the acceleration of the eccentricity both increase in proportion to the square of the rotational speed of the disc. Accordingly, if the acceleration at which the objective lens of the objective lens driver is moving is not sufficiently sensitive to these accelerations, then the objective lens cannot follow any variation in flutter or eccentricity closely, thus causing control errors. In that case, the light beam will be out of focus with the disc surface or go off the predetermined tracks on the disc to possibly deteriorate the quality of a read or write signal.
On the other hand, the optical disc drives are recently required to further reduce their sizes. To meet those demands, the objective lens drivers also need to reduce their sizes (e.g., their thicknesses, in particular). Thus, it has become more and more difficult to obtain an objective lens driver that can exhibit sufficient acceleration sensitivity within a limited space.
Hereinafter, a conventional objective lens driver with a reduced thickness will be described with reference to FIGS. 15, 16, 17A, 17B and 17C. FIG. 15 is an exploded perspective view illustrating the structure of a conventional objective lens driver. FIG. 16 is a plan view of the objective lens driver shown in FIG. 15. FIG. 17B is a schematic plan view showing the positional relationship among magnets, a focusing coil and a tracking coil in the conventional objective lens driver. FIGS. 17A and 17C are transparent plan views of the objective lens driver as respectively viewed in the directions U and V shown in FIG. 17B.
In the conventional objective lens driver, an objective lens 1 is fitted into a lens holder 301 as shown in FIGS. 15 and 16. The lens holder 301 has a central through hole to receive a substantially pentagonal prism focusing coil 302 with a pair of flat and substantially quadrangular prism tracking coils 303. The two tracking coils 303 are connected in series together.
Two magnets 304 and 305 are secured to a base 306 so as to sandwich the focusing coil 302 and the tracking coils 303 with a gap provided between them. A holder 308 is secured to the base 306. A fixing substrate 310 is attached to the back of the holder 308.
A pair of junction terminal plates 309 is secured onto the two side surfaces of the lens holder 301. Four wires 307a, 307b, 307c and 307d are connected to the junction terminal plates 309 such that one end of each of the wires 307a through 307d is soldered up with associated one of the junction terminal plates 309. The other end of the wires 307a through 307d is soldered up with the fixing substrate 310.
Thus, a movable body is made up of the objective lens 1, lens holder 301, focusing coil 302, tracking coils 303 and junction terminal plates 309. That is to say, this movable body is supported by the four wires 307a through 307d so as to be movable both in a focusing direction F and in a tracking direction T with respect to the base 306.
The wires 307a through 307d may be made of an elastic metal material such as beryllium copper or phosphorus bronze, for example. The two terminals of the focusing coil 302 and the four terminals of the pair of serially connected tracking coils 303 are electrically connected to the fixing substrate 310 by way of the junction terminal plates 309 and the wires 307a through 307d. 
Also, as shown in FIG. 16, the magnets 304 and 305 are arranged such that different poles of the magnets 304 and 305 face with other and have substantially the same size J in the tracking direction T.
In the conventional objective lens driver having such a structure, a driving force is generated from portions of the focusing coil 302 and tracking coils 303, which are sandwiched between the magnets 304 and 305. The driving force generating point is located substantially at the center of the movable body. By shifting the location of the objective lens 1 from that driving force generating point, no mechanical interference will occur between a reflective mirror (not shown) for reflecting the light beam in the focusing direction F and the driving means consisting of the magnets 304 and 305, focusing coil 302 and tracking coils 303. In this manner, an objective lens driver with a reduced thickness, which can be used effectively in an optical head with a reduced thickness, is obtained.
Hereinafter, it will be described with reference to FIGS. 17A through 17C how the conventional objective lens driver having such a configuration operates.
First, a focusing drive operation thereof will be described. As shown in FIG. 17A, when a current is supplied to the focusing coil 302 so as to flow in the direction pointed by the arrow If, a driving force is generated in the direction pointed by the arrow Pf along one side of the focusing coil 302 because the opposed magnetic pole is the N pole. As a result, the lens holder 301 is driven in the focusing direction F by the driving force Pf that has been generated in the focusing coil 302.
Next, a tracking drive operation thereof will be described. As shown in FIG. 17C, when a current is supplied to the tracking coils 303 so as to flow in the direction pointed by the arrow It, a driving force is generated in the direction pointed by the arrow Pt along one side of the tracking coils 303 because the opposed magnetic pole is the S pole. As a result, the lens holder 301 is driven in the tracking direction T by the driving force Pt that has been generated in the tracking coils 303.
To allow the conventional objective lens driver to drive the objective lens at an increased acceleration, the following problems must be overcome.
Firstly, in the conventional objective lens driver, the driving force in the focusing direction F relies solely on the driving force that is generated along just one side of the pentagonal prism focusing coil 302. Accordingly, the ratio of the effective length of the focusing coil 302, contributing to the generation of the driving force, to the overall length thereof is very limited. That is to say, the conventional objective lens driver cannot generate the driving force so efficiently.
When the width J of the magnets 304 and 305 is increased to increase the effective length of the coil 302 (when the width J of the magnet 304, located closer to the objective lens 1, is increased, in particular), connecting portions 301a and 301b, which are provided between the objective lens 1 in the lens holder 301 and the focusing and tracking coils 302 and 303, should have a reduced thickness. However, if these connecting portions 301a and 301b have a reduced thickness, then the transmission path of the driving force will have a decreased rigidity. In that case, a high-order resonance frequency of the objective lens driver will decrease and the servo band thereof will also decline, thus deteriorating the ability of the objective lens 1 to follow any variation in the flutter or eccentricity of the disc. Consequently, the quality of the resultant read signal or write signal might deteriorate.
On the other hand, if the width J of the magnet 305 is increased, then the focusing coil 302, surrounding the magnet 305, also needs to have an increased surrounding length. Thus, the ratio of the effective length of the focusing coil 302 to the overall length thereof cannot be increased. Also, even if an increase voltage is applied to the focusing coil 302, the sensitivity of the acceleration generated cannot be increased sufficiently.
Furthermore, in the conventional objective lens driver, just one side of each of the two tracking coils 303 contributes to the generation of the driving force. Thus, the ratio of the effective length of each tracking coil 303 to the overall length thereof cannot be increased, either. For that reason, even if an increase voltage is applied to the tracking coils 303, the sensitivity of the acceleration generated cannot be increased sufficiently.
In order to overcome the problems described above, an object of the present invention is to provide an objective lens driver that can exhibit increased acceleration sensitivity and good follow-up performance even against a high-speed-rotating optical disc and thereby can minimize the unwanted deterioration in quality of the read or write signal.
An objective lens driver according to a preferred embodiment of the present invention preferably includes a movable body, a base, a supporting portion, a first multipolar magnet and a second multipolar magnet. The movable body preferably includes an objective lens to focus a light beam, a lens holder to hold the objective lens thereon, and a coil substrate. The coil substrate preferably includes a focusing coil and at least one tracking coil and is preferably secured onto the lens holder. The supporting portion preferably supports the movable body such that the movable body is movable in a focusing direction and a tracking direction with respect to the base. The focusing direction is parallel to the optical axis of the objective lens, while the tracking direction is perpendicular to the focusing direction. The first and second multipolar magnets are preferably secured to the base so as to sandwich the coil substrate with a gap provided between each of the first and second multipolar magnets and the coil substrate. The focusing and tracking coils are arranged as two flat coils on two mutually parallel separate planes so as to overlap with each other at least partially in a direction that is perpendicular to the focusing and tracking directions.
In one preferred embodiment of the present invention, the first multipolar magnet is preferably provided between the objective lens and the coil substrate.
In a specific preferred embodiment, the focusing coil is preferably located closer to the first multipolar magnet than the tracking coil is, while the tracking coil is preferably located closer to the second multipolar magnet than the focusing coil is.
In another specific preferred embodiment, each of the first and second multipolar magnets is preferably divided into a plurality of magnetic pole regions. The first multipolar magnet preferably includes at least two magnetic pole regions having mutually opposite polarities that are arranged in the focusing direction. The second multipolar magnet preferably includes at least two magnetic pole regions having mutually opposite polarities that are arranged in the tracking direction.
In this particular preferred embodiment, the at least two magnetic pole regions of the second multipolar magnet may include: a first magnetic pole region; and a second magnetic pole region, which has a U-cross section with a flat bottom extending in the tracking direction and which surrounds the first magnetic pole region. The first and second magnetic pole regions are preferably magnetized so as to display mutually opposite polarities.
In an alternative preferred embodiment, the at least two magnetic pole regions of the second multipolar magnet may include six magnetic pole regions obtained by dividing the second multipolar magnet into three columns in the tracking direction and into two rows in the focusing direction. In that case, the six magnetic pole regions are preferably magnetized such that each pair of magnetic pole regions, adjacent to each other in the focusing or tracking direction, displays mutually opposite polarities. One of the six magnetic pole regions, which belongs to the central one of the three columns and to the upper one of the two rows, is preferably used as a first magnetic pole region.
In another preferred embodiment, the first multipolar magnet preferably has the same structure as the second multipolar magnet.
In still another preferred embodiment, the coil substrate preferably includes two tracking coils, including the at least one tracking coil, and the two tracking coils are preferably arranged in the tracking direction.
In yet another preferred embodiment, as measured in the tracking direction, the width M of the first multipolar magnet, the width N of the second multipolar magnet and the width L of the first magnetic pole region preferably satisfy the inequality L less than M less than N.
In yet another preferred embodiment, the at least two magnetic pole regions of the second multipolar magnet may include six magnetic pole regions obtained by dividing the second multipolar magnet into two rows having approximately equal widths in the focusing direction and into three columns in the tracking direction. The widths of the three columns as measured in the tracking direction preferably substantially satisfy a ratio of one to two to one. The six magnetic pole regions are preferably magnetized such that each pair of magnetic pole regions, adjacent to each other in the focusing or tracking direction, displays mutually opposite polarities. On the other hand, the at least two magnetic pole regions of the first multipolar magnet may consist of two magnetic pole regions obtained by dividing the first multipolar magnet into two rows having approximately equal widths in the focusing direction. The two magnetic pole regions are preferably magnetized so as to display mutually opposite polarities.
In this particular preferred embodiment, the first multipolar magnet is preferably almost as tall in the focusing direction as the second multipolar magnet. As measured in the tracking direction, the width of the first multipolar magnet is preferably approximately equal to the width of the two magnetic pole regions belonging to the central column of the second multipolar magnet.
In a specific preferred embodiment, the first multipolar magnet is preferably disposed so as to face the two magnetic pole regions belonging to the central column of the second multipolar magnet.
Specifically, the supporting portion preferably supports the movable body such that the movable body is rotatable on a rotational axis that is defined to be perpendicular to the focusing and tracking directions. The coil substrate preferably includes four tracking coils including the at least one tracking coil. The four tracking coils are preferably arranged symmetrically about a first axis and a second axis. The first axis preferably passes an intersection between the rotational axis and the coil substrate and is preferably defined to be parallel to the focusing direction. The second axis preferably also passes the intersection and is preferably defined to be parallel to the tracking direction.
More specifically, two of the four tracking coils are preferably located over the second axis and connected in series together to form an upper pair of coils, while the two other tracking coils are preferably located under the second axis and connected in series together to form a lower pair of coils.
In this particular preferred embodiment, by supplying currents having the same phase to the upper and lower pairs of coils, the objective lens driver preferably drives the movable body in the tracking direction. By supplying currents having opposite phases to the upper and lower pairs of coils, the objective lens driver preferably rotates the movable body on the rotational axis.
Specifically, the focusing coil is preferably wound around the rotational axis.
An optical head according to a preferred embodiment of the present invention preferably includes: the objective lens driver according to any of the preferred embodiments described above; and a light source for emitting the light beam.
An optical disc drive according to a preferred embodiment of the present invention preferably includes: a motor for rotating an optical disc thereon; the optical head of the present invention, which is disposed at such a position as to form a focal point of the light beam on the optical disc; and means for moving the focal point of the light beam on the optical disc in a radial direction of the optical disc.
Other features, elements, processes, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.